1. Field of the Invention
This invention is generally related to light emitting diode (LED) array head for optical printers, digital copiers or facsimile machines, etc. and is particularly related to edge emitting type LED array heads.
2. Description of the Related Art
A light emitting diode (hereinafter referred to as an LED) array comprises a plurality of end surface emitting LED elements which are disposed in at least one row. Such an LED array is used in an optical printer, a digital copier or a facsimile as disclosed in Japanese Laid Open Patent Nos. 60-32373, 60-90782, 60-90783, 60-90784, 60-99672, 60-99673 and 60-116479.
The LED elements are more reliable for use under vibrations and noises than laser beam printers (LBP). They are also suitable for miniaturizing an optical head since an apparatus with an LED array head does not need a scanning mechanism such as a polygonal mirror as needed by LBP using a semiconductor laser.
On the other hand, the LED elements have at least the following problems. For example, the quantity of light from each element is dispersive, and beam spot shapes are not uniform. These problems generally originate from a unit magnification focusing element of a rod lens array or a gradient index lens etc. rather than the LED itself. In particular, these problems may be caused by the following two reasons: First, a dispersed light loss may occur at connecting portions between the rod lenses. This light loss occurs near the connecting portions due to a difference in a pitch of emitting elements and that of focusing elements. Secondly, it is difficult to uniformly converge light beams since flared light such as reflected light in a rod lens easily occurs.
LED arrays are generally classified into two types. The first type is an area or surface emitting type, and the second type is an edge emitting or end surface emitting type. The emitting type has the following three advantages: First, the edge emitting type is more suitable for an LED array of high density than the surface emitting type. Secondly, the coupling efficiency of a focusing element in the edge emitting type is higher than that of the surface emitting type since the directionality of the output light of the edge emitting type is more focused than that of the surface emitting type. In other words, a diffusion angle is relatively small in a sub-scanning direction. Thirdly, a high output is obtained by a low current.
On the other hand, the end surface emitting type has the following disadvantages: First, when a wire bonding mounting technique as used for an area emitting type LED is applied to the end surface emitting type LED, since the wire bonding has to be performed from an opposite side of the emitting end surface, the end surface emitting type may not be mounted at a high density. Secondly, when chips are cut out from a wafer as shown in FIG. 38, the emitting end of an LED element 4 and the wafer base plate 3 do not share the same plane. The height and width of the cut surface are respectively a few .mu.m and 10 .mu.m. Because of this cut surface, a part of luminous flux emitted from a side of an active layer 2 is reflected on the base plate 3 while other parts of the luminous flux are not reflected on the base plate 3. As a result, a pattern of light emission becomes uneven. Thirdly, when the end surface emitting LED's are combined with rod array, as already discussed above, a disposed light loss occurs at connecting portions between the rod lenses.